1. Field of the Invention
The present invention relates to an imaging optical system, a lens unit, and an image projection apparatus.
2. Description of the Related Art
In a projector that uses a plurality of image panels, optical paths need to be combined before light beams enter a projection lens. When a reflective liquid crystal panel is used, a polarization splitting surface needs to be disposed between the reflective liquid crystal panel and the projection lens in order to separate incident light and emitted light. Conventionally, a projector that is configured by disposing a flat plate as a polarization splitting surface exists. However, when a light beam passes through a flat panel that is disposed so as to be inclined, various kinds of aberrations such as astigmatism are generated and therefore an imaging performance is deteriorated.
Japanese Patent Laid-Open No. H06-337396 discloses a configuration in which two flat plates that have inclination directions orthogonal to each other are used to compensate the astigmatism. Japanese Patent Laid-Open No. H05-100187 discloses a configuration in which a cylinder lens is disposed between a projection lens and an image panel to correct the astigmatism. Japanese Patent Laid-Open No. 2005-189635 discloses a configuration in which a cuneiform element is disposed between a projection lens and an image panel.
According to the configuration of Japanese Patent Laid-Open No. H06-337396, the astigmatism in a cross section along an inclination direction of the flat plate (an inclined angle cross section) can be improved. However, there is a skew ray that is a ray other than the inclination direction of the flat plate in real light beams, and therefore this configuration cannot correct aberration of the skew ray and the imaging performance cannot be sufficiently recovered. Furthermore, Japanese Patent Laid-Open No. H06-337396 discloses a configuration in which two flat plates are disposed in an area of the back focus between the projection lens and the image panel. However, in this configuration, the projection lens needs long back focus and it is undesirable because the weight is heavier along with the enlargement of a lens diameter, the cost is more expensive, and the design performance is deteriorated.
Additionally, in the configuration of Japanese Patent Laid-Open No. H05-100187, since the skew enters the inclined flat plate so as to be asymmetric between the upper side and the lower side, the astigmatism cannot be sufficiently corrected by the cylinder lens that has a shape that is symmetric between the upper side and the lower side.
Moreover, when the cuneiform element is used as disclosed in Japanese Patent Laid-Open No. 2005-189635, the astigmatism and the aberration of the skew ray can be simultaneously improved. However, in the configuration of Japanese Patent Laid-Open No. 2005-189635, the thickness of the cuneiform element is different in accordance with a position where the ray passes. Therefore, an image quality for all image heights cannot be improved since an image plane tilt is generated. Thus, in any cases, the deterioration of the image quality that is caused by the flat plate cannot be sufficiently recovered.